The present invention relates to a liquid crystal shutter or, in particular, to a liquid shutter in which the peripheral circuit or especially the drive circuit can be simplified and the cost and power consumption can be reduced by realizing a low-voltage drive, a high-speed shutter on-off response, a high contrast, etc. The liquid crystal shutter according to this invention has the various advantages as described above, and therefore can find application in various fields such as battery-driven portable equipment, liquid crystal photoprinters and liquid crystal optical elements.
Conventionally, various liquid crystal shutters have been proposed. Their performance, however, is not yet satisfactory, and market demand is high for a liquid crystal shutter for a liquid crystal printer or a liquid crystal optical element high in response speed, bright, high in contrast, drivable by a simple method and capable of a gradated display.
The liquid crystal shutters under development are roughly classified, according to the type of the liquid crystal material used, into (1) a liquid crystal shutter using an ordinary nematic liquid crystal, (2) a liquid crystal shutter using a two-frequency drive nematic liquid crystal having positive or negative dielectric constant depending on the frequency, and (3) a liquid crystal shutter using a ferroelectric liquid crystal having a spontaneous polarization.
The liquid crystal shutter using the ordinary nematic liquid crystal in (1) above is further divided, according to the operating principle, into two well-known types including (a) a TN (twisted nematic) liquid crystal system in which white or black is displayed by use of the property of the optical rotatory power rotating the incident light, and the optical rotatory power is canceled by applying a voltage to the pixels and erecting the liquid crystal molecules at an angle substantially perpendicular to the substrate, and (b) a STN (super twisted nematic) liquid crystal system using a liquid crystal element having a twist angle of 180xc2x0 to 260xc2x0 in which black or white is displayed by applying a voltage to the pixels and thus changing the birefringence which causes a retardation of the incident light.
The prior art having the configuration described in (a) above is disclosed in, for example, Japanese Unexamined Patent Publication No. 62-150330. This conventional system is configured with a TN liquid crystal element having a twist angle of 90xc2x0 and a pair of polarizing plates arranged on the two sides of the liquid crystal element in such a manner the absorption axes thereof are orthogonal to each other.
Another conventional system is disclosed, for example, in Japanese Unexamined Patent Publication No. 9-113864. In this prior art, which uses a liquid crystal element having a twist angle of 270xc2x0, polarizing plates are arranged parallel or perpendicular to the alignment direction of the upper and lower substrates thereby to utilize the optical rotatory power.
The conventional system of (b) described above includes a STN liquid crystal display apparatus called the yellow mode used as an ordinary liquid crystal display apparatus. In the conventional system such as the STN liquid crystal display apparatus having a twist angle of 240xc2x0, for example, the angles of intersection between the absorption axes of the upper and lower polarizing plates is about 60xc2x0, and the angle between the alignment direction of the substrates and the absorption axes of the polarizing plates is set to about 45xc2x0.
In the case where the a twist angle of 90xc2x0 is used in the system of (a) above, the response time required to transfer from the open state to the closed state by applying a voltage is as short as several ms, while the problem is the slow response time of 10 to several tens of ms for returning from the closed state to the open state by removing the voltage. Further, in the case where the twist of 270xc2x0 is used, the orientational stability of the liquid crystal element is difficult to achieve, which makes it impractically necessary to use an obliquely vapor deposited SiO film or the like special alignment layer capable of attaining a high pretilt.
In the system of (b), on the other hand, the use of a practical STN liquid crystal element having the twist angle of 225xc2x0 to 250xc2x0 can attain a short response time of several ms for transferring from closed to open state. In the closed state, however, a voltage is applied to the liquid crystal element to make a bluish black liquid crystal with a low contrast of about 10. In addition, a further increase in applied voltage undesirably increases the brightness again by changing the elliptical polarized state. Therefore, the applied voltage cannot be set to a very high level, with the result that the response time from open to closed state becomes as long as 10 to several tens of ms, thereby posing the problem of poor practicability as a liquid crystal shutter.
Further, based on the aforementioned study of the prior art, the present inventor has proposed a system using a liquid crystal element having a twist angle of 180xc2x0 to 260xc2x0, as disclosed in Japanese Unexamined Patent Publication No. 9-119219, in which the birefringence is utilized for white display as in the STN liquid crystal system while the optical rotatory power is canceled for black display by erecting the liquid crystal molecules substantially perpendicular to the substrate.
Now, the system proposed by the present inventor will be explained with reference to FIGS. 11 and 12. FIG. 12 is a sectional view showing the structure of this liquid crystal shutter, and FIG. 11 is a plan view showing the alignment direction of liquid crystal molecules as viewed from the top in FIG. 12. This liquid crystal shutter, as shown in FIG. 12, has a liquid crystal element 10 having a twist angle of 240xc2x0, a lower polarizing plate 8 and an upper polarizing plate 9.
The liquid crystal element 10 is formed of a first substrate 1 of glass 0.7 mm thick with a first electrode 2 of ITO and an alignment layer 3 formed thereon, a second substrate 4 of glass 0.7 mm thick with a second electrode 5 of ITO providing a transparent electrode and an alignment layer 6 formed thereon, and a nematic liquid crystal 7. The birefringence index xcex94n of the nematic liquid crystal 7 used in this case is 0.2, and the gap d between the first substrate 1 and the second substrate 4 is 4 xcexcm. Therefore, the value xcex94nd indicating the birefringence characteristic of the liquid crystal element 10 is set to 0.8 xcexcm,
The alignment layer 3 associated with the first substrate 1 is rubbed in the alignment direction of the lower liquid crystal molecules located on the boundary surface of the first substrate 1 arranged on the lower side, i.e. in the alignment direction 7b (See FIG. 11) of the lower liquid crystal molecule. The alignment layer 6 of the second substrate 4, on the other hand, is rubbed in the alignment direction of the upper liquid crystal molecules located on the boundary surface of the second substrate 4 located on the upper side, i.e. in the direction 7a (See FIG. 11) of alignment of the upper liquid crystal molecule. Further, a substance having an optically rotatory power called a chiral material is added to the nematic liquid crystal 7 having a viscosity of 18 cp, so that the twist pitch P is adjusted to 8 xcexcm to obtain the relation d/P=0.5 thereby to form a liquid crystal element having a counterclockwise twist angle of 240xc2x0.
Also, the lower polarizing plate 8 and the upper polarizing plate 9 are arranged on the two sides, respectively, of the liquid crystal element 10 in such a manner that the absorption axis 8a of the lower polarizing plate and the absorption axis 9a of the upper polarizing plate are orthogonal to each other. The absorption axis 8a of the lower polarizing plate is arranged at an angle of 45xc2x0 counterclockwise to the direction 12 of the central liquid crystal molecule indicating the alignment direction of liquid crystal at the intermediate portion between the first substrate 1 and the second substrate 4 of the nematic liquid crystal 7. The absorption axis 9a of the upper polarizing plate, on the other hand, is arranged at an angle of 45xc2x0 clockwise to the direction 12 of alignment of the central liquid crystal molecule, thereby making up a liquid crystal shutter of positive type displaying white in the absence of a voltage applied thereto.
As long as no voltage is applied, the linear polarized light entering from the lower polarizing plate 8 is changed to an elliptical plarized light by the birefringence characteristic of the liquid crystal, and, assuming an open state, emitted as white light colored slightly yellowish as compared with the upper polarizing plate 9. Upon application thereto of 20 to 30 V, DC or AC, the liquid crystal molecules are erected in the direction perpendicular to the substrates, and lose both the birefringence characteristic and the optical rotatory power. Thus, the linear polarized light entering by way of the lower polarizing plate 8 proceeds directly through the liquid crystal element and is blocked by the upper polarizing plate 9, thereby changing to a closed state for black display.
The liquid crystal shutter of this type uses a liquid crystal element having a twist angle of 180xc2x0 to 260xc2x0. The response time from closed to open state, therefore, is 1 to 2 ms and much shorter than that of the conventional TN liquid crystal shutter having a twist angle of 90xc2x0 utilizing the optical rotatory power with the polarizing plates arranged parallel to the liquid crystal molecules, thus making possible a high-speed response. The response time from open to closed state, on the other hand, is not more than 1 ms upon application of a high voltage of 20 to 30 V. Further, the absence of the birefringence characteristic in closed state for black display produces a high contrast.
The technique disclosed in Japanese Unexamined Patent Publication No. 9-119219 described above, however, requires a drive voltage of not less than 20 V, and therefore uses a drive IC and a boosting circuit with a high breakdown voltage as a drive circuit.
The object of the present invention is to provide a liquid crystal shutter, which can be driven with a low voltage of not higher than 10 V by reducing the drive voltage thereof further, and in which the on/off switching of the shutter can be performed at high speed while at the same time producing a high contrast, on the one hand, and to reduce the cost and the power consumption by simplifying the drive circuit using the particular liquid crystal shutter, on the other hand.
In order to achieve the object described above, according to a first aspect of the present invention, there is provided a liquid crystal shutter comprising a liquid crystal element holding a nematic liquid crystal having a twist angle of 180xc2x0 to 260xc2x0 between a pair of substrates including a first substrate and a second substrate, a pair of polarizing plates arranged to hold the liquid crystal element described above therebetween, and an optical compensator arranged between the liquid crystal element and one of the polarizing plates, characterized in that the angle formed between the absorption axes of the pair of the polarizing plates is set in the range of 80xc2x0 to 100xc2x0, the angle formed by the absorption axis of one of the polarizing plates and the direction of central liquid crystal molecule of the liquid crystal element is set in the range of 40xc2x0 to 60xc2x0, and the delay axis of the optical compensator is arranged substantially in parallel to the direction of the central liquid crystal molecule of the liquid crystal element.
Also, this aspect of the invention is characterized in that the retardation value on the front of the optical compensator is set in the range of 0.01 to 0.1 m.
Further, this aspect of the invention is characterized in that the value xcex94nd which is the product of the birefringence index xcex94n of the nematic liquid crystal and the gap d between the first substrate and the second substrate is set in the range of 0.6 to 0.9 xcexcm.
Also, this aspect of the invention is characterized in that the optical compensator is a retardation film and the lag axis of the optical compensator has the same direction as the lag axis of the retardation film.
Further, the optical compensator is a discotic film with a discotic structured compound aligned on a transparent film at a varying inclination angle along the thickness thereof, and the delay axis of the optical compensator is an optical axis orthogonal to the alignment direction of the discotic film.
An example of a liquid crystal shutter according to the second aspect of the present invention comprises a liquid crystal element with a nematic liquid crystal having a twist angle of 180xc2x0 to 260xc2x0 held between a pair of substrates including a first substrate and a second substrate, a pair of polarizing plates, a pair of polarizing plates arranged to hold the liquid crystal element, and a first optical compensator and a second optical compensator arranged between the liquid crystal element and one of the polarizing plates, characterized in that the angle formed between the absorption axes of the pair of the polarizing plates is set in the range of 80xc2x0 to 100xc2x0, and the angle formed between the absorption axis of said one of the polarizing plates and the alignment direction of the central liquid crystal molecule of the liquid crystal element is set in the range of 40xc2x0 to 60xc2x0.
Another example of the liquid crystal shutter according to the second aspect comprises a liquid crystal element with a nematic liquid crystal having a twist angle of 180xc2x0 to 260xc2x0 held between a pair of substrates including a first substrate and a second substrate, a pair of polarizing plates arranged with the liquid crystal element held therebetween, a first optical compensator arranged between the liquid crystal element and one of the polarizing plates, and a second optical compensator arranged between the liquid crystal element and the other polarizing plate,
characterized in that the angles formed between the absorption axes of the pair of the polarizing plates is set in the range of 80xc2x0 to 100xc2x0, and the angle formed between the absorption axis of said one of the polarizing plates and the alignment direction of the central liquid crystal molecule of the liquid crystal element is set in the range of 40xc2x0 to 60xc2x0.
Also, this aspect of the invention is characterized in that the difference between the retardation value on the front of the first optical compensator and the retardation value on the front of the second optical compensator is set in the range of 0.01 to 0.1 xcexcm.
Further, this aspect of the invention is characterized in that the value xcex94nd which is the product of the birefringence index xcex94n of the nematic liquid crystal and the gap d between the first substrate and the second substrate is set in the range of 0.6 to 0.9 xcexcm.
Furthermore, this aspect of the invention is characterized in that the value d/P which is the quotient between the twist pitch P of the nematic liquid crystal of a liquid crystal element having a twist angle of xcex8 and the gap d between the first substrate and the second substrate is set in the range of 0.5 to xcex8/360.
Also, this aspect of the invention is characterized in that the first optical compensator is a first retardation film, the second optical compensator is a second retardation film, the delay axis of the first retardation film is arranged substantially orthogonally orthogonally to the direction of the central liquid crystal molecule of the liquid crystal element, and the delay axis of the second retardation film is arranged substantially parallel to the direction of the central liquid crystal molecule of the liquid crystal element.
According to still further aspect of the invention, there is provided liquid crystal shutter, characterized in that the first optical compensator and the second optical compensator are each a discotic film with a discotic structured compound aligned on a transparent film at a varying inclination angle along the thickness thereof, and one of the pair of the polarizing plates makes up an upper polarizing plate, the other polarizing plate makes up a lower polarizing plate, the first optical compensator is a first discotic film, and the first discotic film is interposed between the lower polarizing plate and the liquid crystal element,
characterized in that the direction orthogonal to the direction of the central liquid crystal molecule of the liquid crystal element makes up a horizontal axis, the angle formed between the alignment direction of the first discotic film and the horizontal axis is set in the range of 0xc2x0 to 30xc2x0, the angle formed between the alignment direction of the first discotic film and the alignment direction of the lower liquid crystal molecule is not more than the angle formed between the alignment direction of the first discotic film and the alignment direction of the upper liquid crystal molecule;
the second optical compensator makes up the second discotic film, and the second discotic film is interposed between the upper polarizing plate and the liquid crystal element; and
the angle formed between the alignment direction of the second discotic film and the horizontal axis is set in the range of 0xc2x0 to 30xc2x0, and the angle formed between the alignment direction of the second discotic film and the alignment direction of the upper liquid crystal molecule is not more than the angle formed between the alignment direction of the second discotic film and the alignment direction of the lower liquid crystal molecule.